1 Field of the Invention
This invention relates to a focus detecting device.
2 Description of the Prior Art
A focus detecting device for a single lens reflex camera in which, from a light passed through the focus plate of the single lens reflex camera, a first image and a second image are formed by a pair of re-imaging lenses and the infocus condition of an objective lens is detected from the variations in position of said first and second images with respect to a pair of image position detecting photoelectric converters disposed on or near the focal planes of said pair of re-imaging lenses is already known, for example, from Japanese Laid-open patent application No. 7323/1979.
However, the prior art has the following disadvantages which will hereinafter be described by reference to the accompanying drawings. FIG. 1 is an illustration of the focus detecting device according to the prior art. In FIG. 1, the light beam passed through a phototaking objective lens 1 is converged on the focal plane 2 of a focus plate and then diverged, and part of the light beam passed through the objective lens is converged and imaged on the light-receiving surfaces of a pair of photoelectric elements 4 and 4' by a pair of re-imaging lenses 3 and 3' disposed symmetrically with respect to the optic axis of the objective lens 1. The pair of photoelectric elements 4 and 4' act as image position detecting photoelectric converters and specifically comprise a photoelectric element array. It should be noted here that the distributions of the intensities of illumination on the light-receiving surfaces of the photoelectric elements 4 and 4' for an object must be equal to each other. This is because the focus detection is accomplished by comparing the photoelectric outputs for the same region of an image (light image).
Now, when light beams impinging on the upper light-receiving surface 4a, the central light-receiving surface 4c and the lower light-receiving surface 4b of the photoelectric element 4 are conversely projected upon the exit pupil of the objective lens 1 from these light-receiving surfaces through the re-imaging lens 3, they become such as shown in FIG. 1. That is, a light beam forming an opening a.sub.1 a.sub.2 a.sub.3 impinges on the upper light-receiving surface 4a, and a light beam forming an opening c.sub.1 c.sub.2 c.sub.3 impinges on the central light-receiving surface 4c. These openings a.sub.1 a.sub.2 a.sub.3 and c.sub.1 c.sub.2 c.sub.3 are determined by the effective F-number of the re-imaging lens 3 and are substantially equal in size. However, when converse projection is effected upon the lower light-receiving surface 4b in the same manner as upon the former two light-receiving surfaces, the light beam which should impinge on the light-receiving surface 4b through the re-imaging lens 3 has an opening b.sub.1 b.sub.2 b.sub.3, but when the pupil diameter of the objective lens is small, part of the light beam is missed at the upper end of the objective lens 1 and only a light beam forming an opening b'.sub.1 b.sub.2 b.sub.3 can impinge on the lower light-receiving surface 4b. This means that, for example, when an object having a uniform distribution of brightness is measured, the intensity of illumination on the lower light-receiving surface 4b is reduced as compared with the intensities of illumination on the upper light-receiving surface 4a and the central light-receiving surface 4c. That is, a distribution of intensity of illumination corresponding to the brightness distribution of the object cannot be obtained on the light-receiving surface of the photoelectric element 4.
This phenomenon equally occurs to the other photoelectric element 4' and the intensity of illumination on the upper light-receiving surface 4'a on which the light beam missed at the lower end of the objective lens 1 impinges is reduced below the intensities of illumination on the central light-receiving surface 4'c and the lower light-receiving surface 4'b. Now, the light image formed on the light-receiving surface 4b and the light image formed on the light-receiving surface 4'a are identical, but the reproducibility of the intensity of illumination of the light image for the object differs from region to region and therefore, the distributions of brightness of the images on the photoelectric elements 4 and 4' do not become identical. Accordingly, the photoelectric outputs of the photoelectric elements 4 and 4' cannot be properly compared and thus, there occurs a focus detection error.